The present invention relates generally to detection and remediation of potentially hazardous conditions in an electric vehicle battery enclosure, and more particularly but not exclusively, to detecting and correcting high voltage electrolysis within the battery enclosure to limit possible excessive thermal conditions of the individual battery cells and modules.
Battery packs used with electric vehicles store large amounts of energy in a small space, producing high energy densities. These battery packs include an external housing that is designed for more than just environmental protection and packaging efficiency. The housing also enhances safety and stability, particularly under a range of anticipated abnormal operating conditions.
The battery packs are designed to provide high levels of safety and stability, yet situations can arise where a portion of a battery pack experiences a local thermal condition which generates significant heat. When the temperature is great enough and sustained, the local thermal condition can transform into a runaway thermal condition affecting wide areas of the battery pack, and sometimes the entire battery pack under certain circumstances. This runaway thermal condition can begin when the local thermal condition approaches the runaway thermal point, which can be about 200° C.
Battery pack designs include an integrated and isolated cooling system that routes coolant throughout the enclosure. When in good working order, the coolant from the cooling system does not come into contact with the electric potentials protected within. It does happen that sometimes a leak occurs and coolant enters into unintended parts of the enclosure. In certain situations, the coolant may be electrically conductive and can bridge terminals having relatively large potential differences. That bridging may start an electrolysis process in which the coolant is electrolyzed and the coolant will begin to boil when enough energy is conducted into the electrolysis. This boiling can create the local thermal condition that can lead to the runaway thermal condition described above.
The specific boiling point is dependent upon the characteristics of the coolant. For example, one type of coolant includes a mixture of ethylene-glycol and water which can boil at around 107° C. If enough coolant is in the enclosure and/or enough energy is used in electrolyzing the coolant, the boiling point rises due to chemical changes in the coolant from the localized boiling. In other words, the boiling of the coolant changes the boiling point. (For example, as the percentage of water is reduced in a glycol/water coolant mixture from the boiling, the boiling point increases. Pure ethylene-glycol has a boiling point of about 197° C.) As energy continues to be diverted into the electrolysis, the temperature of the local thermal condition rises and will reach close to the thermal runaway temperature if the electrolysis continues long enough under particular circumstances. Surfaces of the battery pack in contact with liquids at this temperature may initiate the runaway thermal condition.
The increasingly elevated temperature from the boiling coolant can therefore be great enough, depending upon many factors including the temperature and amount of coolant and location of the electrolysis, to initiate the runaway thermal condition. The chain reaction results from a heating of adjacent cells, which can cause them to overheat and fail, releasing heat that, in turn, propagates throughout the expanding affected area.
Once the reaction starts, it can continue to spread throughout the battery pack or a portion thereof until overheating cells are sufficiently cooled or the entire battery pack or the portion is consumed. A typical battery pack has a high thermal mass, mostly due to the mass of the cells. A failure of an individual cell provides for a relatively low energy release. Also, surrounding battery cells must be heated to as much as 200° C. or higher before they in turn release energy. These three factors mean that a full reaction that consumes all the cells of a battery pack may take anywhere from tens of minutes to many hours.
While the risk of the runaway thermal condition can be significant in cases of high-voltage electrolysis, another risk can develop as well. This risk is a consequence of the chemical reaction of electrolysis in which hydrogen gas is generated. Under some conditions, the hydrogen gas released during electrolysis can build-up inside the battery pack and become a possible hazard.
A conventional solution for a problem of an initiated chain reaction is to simply permit the reaction to run its course once passengers and bystanders are clear of the vehicle. This is because it can be difficult to safely stop the thermal reaction.
What is needed is an apparatus and method for detecting and responding to high voltage electrolysis within an electric vehicle battery enclosure to limit possible excessive thermal condition of the individual battery cells and modules.